A conventional motor drive apparatus has a plurality of inverter circuits, which supply electric power to coil sets of a motor. When any one of the inverter circuits or any one of the coils sets fails, the motor drive apparatus continues power supply to the motor by only other inverter circuits operating normally.
For example, according to a motor drive apparatus for an electric power steering system disclosed in JP-A-2011-131860 (US 2011/0156629 A1), power supply relays are provided in power supply branch paths connecting a DC battery and inverter circuits of two power supply systems. When either one of the inverter circuits fails, one power supply relay connected to the inverter circuit of the power supply system, which is failing, is turned off to interrupt the power supply and the motor is driven by only the other inverter circuit of the power supply system, which is operating normally.
Such a relay as used for interrupting the power supply from the battery to the inverter circuit may be a mechanical relay or a semiconductor relay. The semiconductor relay is used in more instances in view of space restraint and cost restraint. As the semiconductor relay, a MOSFET is used in more instances in a 12V-power supply system in view of its on-resistance and saturation voltage loss. The MOSFET has a parasitic diode. In view of a possibility that a power source is connected in reverse polarity through error, two MOSFETs are connected in series so that flow directions of free-wheeling currents of respective parasitic diodes are opposite to each other.
A motor generally operates as a generator, which generates an induced voltage when its rotary shaft is rotated by an external force. In an electric power steering system of a vehicle, for example, a motor is rotated by an external force when a tire wheel of a vehicle hits and runs on an obstacle and the like.
In this case, the energy of the induced voltage is regenerated to a battery from an inverter circuit through a power supply relay in a power supply system, in which the power supply relay is in the turned-on state. In a power supply system, which is failing, a power supply relay is turned off and all switching elements in the inverter circuit are turned off. Thus no current path for regenerating the induced voltage is provided. The induced voltage thus becomes excessively high and possibly breaks down circuit elements.